The present invention relates generally to cooling systems for heat-generating devices and, more particularly, to a spray cooling system and a method of using the spray cooling system to cool semiconductor devices.
With the advent of semiconductor devices having increasingly large component densities, the removal of heat generated by the devices has become an increasingly challenging technical issue. Extrapolating the ongoing changes in microprocessor organization and device miniaturization, one can project future power dissipation requirements of 100 W from a 1 cm by 1 cm core CPU surface area within the easily foreseeable future.
Furthermore, as semiconductor devices are designed with larger and larger component densities, greater numbers of functions are being designed into single semiconductor chips. For example, chips are presently available that carry multiple CPU modules along with a cache to support these CPUs. Chips can include numerous modules, such as application-specific integrated circuits (ICs), various forms of random access memory (RAM), and dc-dc converters. Each module on a chip can have different types of components, different component densities, and different times of high activity. Thus, each module can have significantly different power dissipation requirements from the others, with each module""s dissipation requirements independently varying over time.
In the past, the low power dissipation of most chips accommodated the use of low cost, air-cooled heat sinks, which did not typically need to account for local differences in dissipation requirements across a chip. However, higher dissipation chips require substantially greater dissipation than air-cooled heat sinks could reasonably provide. Other known cooling methods for semiconductors include free-flowing and forced-liquid convection, pool boiling (i.e., boiling a liquid cooling fluid off a submerged device), and spray cooling (i.e., boiling a liquid cooling fluid off a device being sprayed with the liquid). Because liquids typically have a high latent heat of vaporization, these latter two methods provide for high heat-transfer efficiency, absorbing a large quantity of heat at a constant temperature.
Pool boiling is limited to a maximum power density, its critical heat flux (CHF). At higher densities, the vaporized cooling fluid forms a vapor barrier insulating the device from the liquid cooling fluid, thus allowing the wall temperature of the device to increase greatly. Spray cooling is also limited to a maximum power density, however, its CHF can be well over an order of magnitude higher than the CHF of a pool boiling system.
Critical to consistent, controlled spray cooling is the controlled application of the liquid cooling fluid in a desired flow rate, and velocity. To reach dissipation levels nearing the spray cooling CHF, the cooling fluid must be sprayed at a correct mass flow rate. At such a rate, vaporization occurs in the nucleate-boiling regime (i.e., the cooling fluid forms a thin film on the chip that vaporizes immediately). If the sprayer""s mass flow rate is decreased below a minimum level for the nucleate boiling regime, then inadequate cooling fluid is available to dissipate the heat and it surpasses the spray cooling CHF (for that cooling fluid mass flow rate), and thus the cooling enters a dry-out regime where the chip""s wall temperature increases substantially until radiant heat can dissipate the generated heat.
If, as the sprayer""s mass flow rate is increased above a maximum level for the nucleate boiling regime, cooling fluid begins to pool on the chip, it enters a pool-boiling regime, which is equivalent to immersion and has substantially lower dissipation levels than the nucleate boiling regime. Thus, the temperature rises substantially until the necessary dissipation level is reached, either in nucleate boiling or radiant heating. Therefore, it is important for spray cooling to be conducted at a proper mass flow rate, maintaining a nucleate boiling regime (or close to it). This makes critical the design of the sprayer, i.e., the design of the nozzle and its related spray devices.
With reference to FIG. 1, in spray cooling, an inert spray coolant from a reservoir 11 is uniformly sprayed by a group of one or more sprayers 13 onto an aligned group of one or more chips 15 mounted on a printed circuit board 17. The coolant preferably evaporates, dissipating heat within the chip. The sprayers and chips, and the board, are mounted within sealed cases 19 fixed within an electronic device such as a computer system. The sprayed coolant is typically gathered and cooled within a condenser 21, and then routed back to the reservoir by a pump 23. The distance between the sprayers and the chips, and the mass flow rates for the sprayers, are typically selected based upon the chip with the highest dissipation requirements.
Typically, the cooling fluid used for spray cooling has a relatively low boiling point (the temperature to maintain) and is inert to the heat source. For semiconductor devices, low boiling point fluids such as 3M FC-72 (FED. CIR.-72, i.e., FLUORINERT(copyright), sold by 3M Corporation) or PF-5060 are among a number of known suitable cooling liquids.
Current sprayer designs commonly employ either pressurized liquid spraying (i.e., pressure-assisted spraying) or pressurized gas atomizing. Because these devices are difficult to control, they are limited in their ability to limit xe2x80x9cpoolingxe2x80x9d of the fluid (i.e., building up of liquid on the cooled device due to excessive spray rates). These cooling configurations typically spray a uniform or uncontrollably varied distribution of coolant across each chip.
However, higher dissipation chips can have substantially greater differences in dissipation across the chip, and as a result can develop detrimental thermal gradients. Additionally, spray cooling systems can develop efficiency problems when uniformly spraying chips having large dissipation variances. In particular, pooling can occur on one portion of a chip when it receives the required level of spray to cool a hotter portion of the chip. As a result, not only can the lower dissipation section end up operating at a significantly higher temperature, but the excess cooling fluid can run from one portion to another, causing further cooling problems. Thus, spray cooling systems can be very sensitive to dissipation rate gradients across the surface of a semiconductor chip.
Accordingly, there has existed a need for an easily maintainable spray cooling system that maximizes spray cooling efficiency for components having complex dissipation requirements, such as may be caused by providing numerous modules of different types within a semiconductor chip. This system preferably can be adapted for a variety of chips, and electronic systems using the cooling system preferably can be easily adapted to chip upgrades. Preferred embodiments of the present invention satisfy these and other needs, and provide further related advantages.
In various embodiments, the present invention solves some or all of the needs mentioned above by providing a cooling system that efficiently operates on one or possibly more high-dissipation devices, providing cooling to various regions of the devices at various dissipation rates.
A cooling system of the invention is typically configured for cooling a device having two or more regions characterized by different thermal dissipation rates. The system includes a sprayer head having one or more sprayers targeted to spray cooling fluid substantially at the first region of the device, and one or more sprayers targeted to spray cooling fluid substantially at the second region of the device. The invention features a control system configured to separately control the sprayers targeting the first and second regions such that the first region is sprayed with a cooling-fluid mass flow rate appropriate for dissipating the thermal energy of the first region at the first thermal dissipation rate, and such that the second region is sprayed with a cooling-fluid mass flow rate appropriate for dissipating the thermal energy of the second region at the second thermal dissipation rate.
This feature of the invention provides for the different regions of the device to be cooled at dissipation rates appropriate to their needs. In some instances, this will prevent local overheating that could occur from pooling or dry-out on one portion of the device.
The cooling system also features a detection system configured to separately detect whether pooling is occurring on one or more regions of the device. Using this feature, accurate determination of the appropriate spray rate can be ascertained, providing for optimal cooling rates over each region.
The cooling system further features a housing containing the sprayer head. The housing is configured such that, when the device is mounted on a substrate, the housing can adjoin the substrate to form a spray chamber containing the device. Advantageously, this feature provides for the cooling system and device to be combined into a compact and easily installable unit. As a result, electronic device manufacturers do not need to provide separate attention to sprayer assemblies, as they come attached to the device.
A method of the invention, for cooling a device having two or more regions characterized by separate and distinct thermal dissipation rates, comprises spraying the different regions of the device with cooling fluid. It further includes controlling the spraying of the regions such that cooling fluid is sprayed on each region at a mass flow rate appropriate for dissipating thermal energy from the region at the appropriate thermal dissipation rate.
Other features and advantages of the invention will become apparent from the following detailed description of the preferred embodiments, taken with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The detailed description of particular preferred embodiments, as set out below to enable one to build and use an embodiment of the invention, are not intended to limit the enumerated claims, but rather, they are intended to serve as particular examples of the claimed invention.